The complex array of proteins on the surfaces of bacterial pathogens can be regarded as organelles that permit pathogens to sense and interact with their hosts. Because expression of specific cell surface proteins is often critical for pathogenesis, small molecules that bind such structures are potentially useful as antimicrobial agents. Small molecules could sterically block sites for binding of host macromolecules and/or disrupt the secondary structures of the bacterial proteins. We propose to test this hypothesis using the M protein of Streptococcus pyogenes as a model target protein. S. pyogenes is a Gram-positive bacterium capable of causing a wide range of diseases in humans. M protein is perhaps the most well characterized virulence factor of S. pyogenes. M protein is a multi-functional virulence factor involved in adherence, intracellular invasion, resistance to phagocytosis, and auto-aggregation of streptococci. Although there are over 100 known serotypes of M, the extracellular portions of all M proteins are presented on the cell surface as alpha helical, coiled-coil dimers. Many activities of M protein depend on binding of host plasma or extracellular matrix proteins. In several cases, the coiled-coil conformation is required for high affinity binding of plasma proteins by M. We intend to initiate the development of anti-microbials that specifically target the M protein of streptococci. First, we will identify small peptides that bind specifically to M. The capacities of the synthetic peptides to affect: 1) M binding to purified plasma proteins; 2) streptococcal adherence to and invasion of human epithelial cells, and 3) phagocytosis of M+ bacteria will be determined. Peptides found to inhibit one or more functions of M will be tested for efficacy in inhibiting the replication of streptococci injected into the peritonea of mice. Peptides that inhibit streptococcal replication in vivo will serve as lead compounds to identify nonpeptide antagonists of M protein. This study may eventually lead to the development of novel compounds for the treatment of S. pyogenes infections. Moreover, this study will help define the relationship between M protein structure and function and aid in understanding the roles of M protein in GAS pathogenesis.